1. Field of the Invention
The present invention relates to a laser beam scanning optical apparatus used as a writing optical system for use in a laser beam printer (LBP) and a digital plain paper copier (PPC) and to a light source apparatus for use in the scanning optical apparatus, and more particularly, to a multibeam scanning optical apparatus, for simultaneously scanning a plurality of lines in parallel by use of a plurality of laser beams and to a laser light source apparatus for use in the multibeam scanning optical apparatus.
2. Description of the Prior Art
Conventionally, in the field of laser beam scanning optical apparatus, multibeam scanning optical apparatuses have been developed where a plurality of scanning lines are simultaneously scanned by use of a plurality of laser beams to substantially increase the speed of image formation substantially with the same number of rotations of the deflector, i.e. with the same speed of scanning in the main scanning direction and where a plurality of laser beams successively scan the same single scanning line to increase the density of images formed on the surface to be scanned (hereinafter, referred to as scanned surface).
In recent years, to meet the demand for laser beam scanning optical apparatuses to form high-resolution images, another type of multibeam scanning optical apparatus has been developed where by using a plurality of laser beams, the distances between the scanning lines are reduced to be shorter than those of an apparatus using a single laser beam so that images of higher resolution are formed substantially at the same speed.
There is a prior art of the multibeam scanning optical apparatus (hereinafter, referred to as first prior art) where a plurality of laser beams irradiated from a plurality of semiconductor lasers are shaped through a beam splitter into a plurality of luminous fluxes whose optical axes are close to one another and which advance in the same direction, and the luminous fluxes are directed to the scanned surface to simultaneously scan a plurality of scanning lines.
FIG. 5 shows a schematic arrangement of the first prior art which is provided with two semiconductor lasers 1 and 1' as the plurality of semiconductor lasers. In this arrangement, the semiconductor laser 1' irradiating a laser beam reflected by a beam splitter 2 is typically disposed in either of a position A such that the laser beam is incident on the beam splitter 2 from the sub scanning direction and a position B such that the laser beam is incident on the beam splitter 2 from within the main scanning plane.
The laser beam irradiated from the semiconductor laser 1 is shaped into a parallel luminous flux by a condenser lens 301 and then, transmitted by the beam splitter 2 without changing its direction. On the other hand, whether the semiconductor laser 1' is disposed in the position A or in the position B, the laser beam irradiated therefrom is shaped into a parallel luminous flux by a condenser lens 301' and then, incident on the beam splitter 2 to be reflected by an interference film (not shown) provided in the beam splitter 2, so that its direction is the same as that of the laser beam transmitted by the beam splitter 2.
In FIG. 5, the two laser beams are depicted such that their optical axes coincide with each other after having exited from the beam splitter 2. In actuality, however, the optical axes are two substantially parallel lines which are close to each other. The two laser beams are directed along substantially the same optical path to the scanned surface provided on a photoreceptor drum 37, where they are imaged into two spots with a predetermined distance therebetween in the sub scanning direction to thereby scan two scanning lines simultaneously.
For that purpose, the following elements are disposed on the optical path along which the two laser beams having exited from the beam splitter 2 are directed: a polygonal mirror 31 deflecting the two laser beams in the main scanning direction at a uniform angular velocity; an f.theta. lens 34 for causing the laser beams deflected by the polygonal mirror 31 to scan the scanned surface at a uniform speed; and first and second cylindrical lens units 30 and 36 which cooperatively correct an inclination of the deflecting surface of the polygonal mirror 31.
There is another prior art of the multibeam scanning optical apparatus (hereinafter, referred to as second prior art) where as shown in FIG. 6, an array-form semiconductor laser 101 is used as the light source. In this arrangement, a plurality of laser beams irradiated from array elements 101a, 101b and 101c are directed to the scanned surface and imaged into different spots by a scanning optical system similar to that of the above-described first prior art, so that a plurality of lines are scanned simultaneously.
Problems faced by the prior arts will be described. In the multibeam scanning optical apparatus according to the first prior art, even if the relative positions of the two spots into which the two laser beams are imaged on the scanned surface are initially adjusted, the relative positions of the semiconductor lasers 1 and 1', the condenser lenses 301 and 301' and the beam splitter 2 sometimes shift due to stress received when the optical elements are mounted in the apparatus body and thermal expansion caused by a temperature change. When this happens, angle errors are caused in the irradiation directions of the laser beams. As a result, the relative positions of the two spots into which the laser beams are imaged on the scanned surface shift over time.
If the relative positions of the two spots into which the two laser beams are imaged on the scanned surface shift in the main scanning direction or in the sub scanning direction, non-uniformity is caused in the main scanning direction at the start points of the scanning lines, or non-uniformity in the pitches of the scanning lines is caused in the sub scanning direction, so that the image quality deteriorates.
Especially in the first prior art, since the condenser lenses 301 and 301' are disposed between the semiconductor lasers 1 and 1' and the beam splitter 2 as shown in FIG. 5, if the positions of the semiconductor lasers 1 and 1' held integrally with the condenser lenses 301 and 301' shift relative to the position of the beam splitter 2, the relative positions of the two spots simultaneously formed on the scanned surface greatly shift.
In the multibeam scanning optical apparatus according to the second prior art shown in FIG. 6, where each of the array elements 101a, 101b and 101c constituting the semiconductor laser array 101 generates heat, if the heat is mutually transmitted, the temperatures of the array elements are affected by the heat, so that the wavelengths and quantities of the laser beams irradiated by the array elements 101a, 101b and 101c vary.
To avoid this problem, it is necessary to increase the distances between the array elements 101a, 101b and 101c. To do so, however, it is necessary to reduce the overall magnification of the optical system. If the magnification is reduced, the focal length of the scanning lens is reduced. Then, only a small space is left for disposing optical elements such as the polygonal mirror. Thus, such an arrangement is impractical.